Twin coil electromagnetic actuator



Sept. 8, 1959 L. E. LAWRENCE TWIN COIL ELECTROMAGNETIC ACTUATOR 5 Sheets-Sheet 1 Filed July 30, 1956 INVENTOR I a. 4 ATTORNEY 5 Sheets-Sheet 2 INVENTOR a. 7 ATTORNEY p 1959 L. E. LAWRENCE TWIN COIL ELECTROMAGNETIC ACTUATOR Filed July 30, 1956 o 3 4 H m J my, 8 a m I k /r 67 14A 7 a0 2 MW z Z fi PQ S 3 a0 filfl l -u -4 I Z2 4 5 iv 7 m 00 7 Z 6 5 /9 z 0 0 M a 5 a 5 l l1l|||1 5 I0 5 7 $0.. a a .7 2 M 7 all 6 3 O 6 4' z a 3 p 1959 L. E. LAWRENCE 2,903,537

TWIN COIL ELECTROMAGNETIC ACTUATOR Filed July 30, 1956 3 Sheets-Sheet 3 I Wi INVENTOR ATTORNEY 1 2,903,537 TWIN COIL ELECTROMAGNETIC ACTUATOR Leland E. Lawrence, Wauwatosa, Wis., assiguor to Allen- Bradley Company, Milwaukee, Wis., a corporation of Wisconsin 7 Application July 30, 1956, Serial No. 600,978

15 Claims. (Cl. 200-104) This invention relates to electrical switches of the electromagnetically actuated type and it resides more particularly' in a switch of this character in which the magnet parts, subject to heating, abut surfaces of a heat conducti-ve mounting frame or integral extensions thereof formed in heat conveying thicknesses from one of the more highly heat conductive metals and in which, in certain forms, the extensions of the frame act to furnish upper and lower guiding surfaces for movable parts of the magnet located above and below the working gap of the magnet; the magnet also, at times, employing an armature guided and held by the guiding means in substantially parallel attitude with respect to the yoke; the switch load and guiding means being symmetrically related to the armature to facilitate employment of a double working gap in the magnet Without danger of development of harmful, eccentric, attractive forces.

Among the objects of this invention is the aim of providing an electromagnetically actuated switch of increased compactness by employing a magnet in which the materials contained are more heavily worked electrically and magnetically but in which excessive temperature rise is avoided by more rapidly extracting heat from the heavily worked materials.

Another object of this invention is to provide an electromagnetically operated switch in which the magnet temperature remains low even though a heavily worked double working gap magnet, with characteristically small heat dissipating surface, is employed.

Another object of this invention is to provide an electromagnetically actuated switch in which the movable contacts and armature are adequately guided in their movement by guiding surfaces above and below the working gaps of the magnet even though an effective part of the magnet surface abuts a heat sink member.

Another object is to provide positive retention of substantially parallel attitude between armature and yoke in a double working gap magnet during the working stroke and until sealing contact is made to avoid inequality in attractive forces at the two working gaps.

Another object is to provide dimensional and form stability in an electromagnetically excited laminated iron structure without clamping stresses strong enough to impair the eifective electrical insulation of existing oxide films between larninations by capillary introduction into the interlamellar voids of a liquid thermosetting insulating bonding agent having high wetting power and, thereafter, subjecting the structure to temperatures sufficient to cure the bonding agent.

A further object is to provide an electromagnetically excited iron structure employing embedded low resistance shading coils, or conductors, in which current is to be induced wherein the embedment is rendered secure and electrical insulation is improved by capillary introduction into the interface between the iron and the low resistance conductor of a liquid, thermosetting, insulatirig bonding agent having high wetting power and, thereafter, subjecting the structure to temperatures sufficient to cure the bonding agent.

The foregoing and other objects and advantages of this invention will appear from the description following, which is set forth by reference to the accompanying drawings, in which there is set forth by way of illustration and ice not of limitation, a form in which this invention may be embodied.

In the drawings:

Fig. 1 is a front view in elevation with parts broken away and in section showing one form of the switch of this invention,

Fig. 2 is a side view in elevation of the switch moved to closed position,

Fig. 3 is a detailed, fragmentary view in front elevation and with parts removed and broken away showing the magnet parts of the switch,

.the same.

Fig. 4 is a side view in elevation and in section of the switch viewed through the plane 44 indicated in Fig. 1,

Figs. 5, 6 and 7 are top plan views in section of the switch viewed, respectively, through the planes 5--5, 6-6 and 7-7 indicated in Fig. 3,

Fig. 8 is a plan view in section viewed through the plane 8-8 indicated in Fig. 3,

Fig. 9 is an enlarged fragmentary view in detail showing the embedment of the shading coil of the switch,

Fig. 10 is a perspective view of the heat accepting frame upon which the switch is mounted, and

Fig. 11 is a detailed side view in section viewed through the plane 1111 indicated in Fig. 3.

The frame As shown in the drawings, a switch, constructed in accordance with this invention, may include a frame 1 which serves not only as a mounting for the several' parts but which may also act to extract heat from the switch magnet parts subject to heating, which heat, in turn, is dissipated over the larger surface area of the frame. It appears in Figs. 4 to 7 and in Fig. 10 that the frame 1 may be formed with relatively thick sections in its major parts, and this, combined with the use of metal of high thermal conductivity, renders the frame effective for transmission of heat at increased rates. It is preferred that a light metal, such as aluminum, having high thermal conductivity, be employed for this purpose in order to avoid excessive weight. Since it is rapid acceptance of heat that is desired, some reduction in bulk but not of weight may be obtained through the use of copper or other material having greater thermal conductivity than aluminum.

As may be observed, particularly in Fig. 10, the frame 1 includes integral extensions 2, 3, 4 and 5, which, as

will he later explained, perform guiding and mounting functions. The extensions 2, 3, 4 and 5 further perform the function of thermal conductors reaching into the parts of the structure where heat originates for extraction of Being integrally formed with the frame 1, resistance to flow of heat across a joining interface is avoided.

The frame 1 also includes a well 6 in which one end of a coil assembly may be socketed in a manner hereinafter to be described. The well 6 presents a substantial area of heat accepting surface to the coil assembly socketed therein, thus facilitating removal of heat therefrom.

Between the extensions 2 and 3 at the inner ends thereof,

the frame is further recessed as indicated at 7 in Fig. 10 for a similar purpose.

The switch hood Secured to the upper part of the frame 1 is a switch hood assembly 8 made up of a stationary contact mountproducts. r r

'The assembly 8 is held in place against a shelf 12 formed near the upper end of the frame 1 by screws13 shown in dotted outline in Fig. 2. The screws 13, extending downwardly through openings in the mounting 9, pass through openings in the inner closure 11 as well and are then received and secured in tapped holes provided therefor in the shelf '12. The outer closure engages the mounting 9 with an interfitting joint 14 and is secured by screws 15 which extend through troughs provided therefor in the mounting 9 to engagement with tapped holes located to receive them in the frame 3.

Extending downwardly through the top of mounting 9 is a set of stationary contact studs '16 anchored by diagonal anchorage screws 17 which hold the heads of stationary contacts 18 securely against the lower surface of mounting 9. In the switch shown four pairs of stationary contacts 18 are employed and arranged in individual arc isolating pockets formed in the mounting 9 and completed by the closures N and 11.

Located to cooperate with the stationary contacts 18 are corresponding sets of movable contacts 19 mounted on the upper surfaces of the ends of conductive bridge bars 28. The bridge bars 20 are guided by the sides of cage assemblies 23 urged upwardly by spring caps 21 bearing against the tops of expansion springs 22 seated, as shown, within the guiding stirrups. In this way the bridge bars 28 are urged upwardly so as to bear against the upper retaining cross portions 24 of the stirrups, when the switch is in open position as shown in Fig. 1. When the switch moves to closed position, as indicated in broken lines in Fig. 2, the bridge bars 20 depart from the retaining portions 24 to transfer the expansive force of the springs 22 to the stationary contacts 18.

The sliding carrier The movable contact assemblies 23 are carried through a short translatory up and down movement between open and closed position by a sliding carrier 26 which includes an upper horizontal contact bar 27 upon which the contact assemblies 23 are mounted by means of screws 28 and nuts, as shown in Fig. 4. The carrier 26 may be made up of a single piece plastic molding from material having good insulating properties and sufficient strength. Because impact strength and dry bearing properties are both important, a phenolic resin reinforced with high strength fibre floc has been found especially useful in forming carrier 26.

Extending downwardly from and integrally joined to contact bar 27 is a pair of side arms 29-29 having upper lateral slide surfaces 3tl-30 facing the outer sides of extensions 2 and 3, in sliding contact therewith, and lower lateral slide surfaces 31-31 facing the inner sides of extensions 4 and 5.

In this way, lateral tilting of the carrier 26 is controlled by the substantial vertical spacing of the upper and lower guiding surfaces. Sliding contact further occurs over a considerable bearing area between the molded material of carrier 26 and the metal of extensions 2, 3, 4 and 5 so that excellent dry bearing conditions exist.

Tilting of the carrier 26 inwardly and outwardly is similarly restrained through engagement, in dry bearing, sliding contact, of upper and lower inner slide surfaces 3232 and 3333 with guide ways 3636 formed as a part of frame 1 and upper and lower outer slide surfaces 3434 and 35-35 with the inwardly facing guide Ways 37 formed on the inner face of cap piece 38. The cap piece 38, shaped by molding, is provided with tenons which extend inwardly, as shown in Figs. 5 and 7, to engage mortises 39-39 and 40-40 in the ends of extensions "2, 3, 4 and 5. Screws 41-41, extending through the cap piece 38, engage threaded holes 4242 in the ends of extensions 2 and 3 and thus, 'with the locating action of the tenons, secure the cap piece 38 in position.

The spaced lateral and inner and outer guiding surfaces thus confine the carrier to vertical translatory sliding motion without binding and with little tilting.

The magnet The magnet which furnishes the motor action for closing the switch includes an iron yoke 43 and an iron armature 44, each laminated and of roughly U shape as appears more clearly in Figs. 4 and 5. The iron parts are excited by dual windings contained in coil assembly 45 located to cause the parts to cooperate in a manner consistent with principles disclosed in my copending application for United States patent, Serial No. 455,695, filed September 13, 1954 for Short Stroke Electromagnetic Actuator.

The yoke 43 consists of a plurality of magnetic laminations having the profile, shown in Fig. 4, clampedbetween side plates by rivets as shown. The inner and outer vertical faces of the yoke 43 are notched respectively to receive'a supporting ledge 46 staked into the frame 1 as appears and a ledge 47 formed as a part of cap piece 38. A small clearance is permitted between the notches and the ledges 46 and 47 sufficient to permit the yoke 43 to make small accommodating movements when the magnet comes to closed position. It is preferred also that the ledges 46 and 47 be of a composition that is sufliciently yieldable to moderate the peak impact stresses occurring on closing. In some cases where the magnet is of such size as to justify it, ledges 46 and 47 may be of such composition as will absorb and minimize the transmission 'of commercial power frequency vibrations.

The lower ends of the legs of the yoke 43 provide pole faces and these are slotted to receive shading coils or short circuit loops 48 embedded as shown. It is preferred that the conductors or loops forming the shading coils 48 be embedded and retained by a thermosetting resin cement 49 as shown in greater detail in Fig. 9, but which, as indicated in Fig. 4, does not fill the slots in the pole faces.

Cements suitable for the purpose include the so-called epoxy resin cements which may be applied in liquid form or in solid form and melted and then cured. The high degree of fluidity of these cements and the strong wetting properties which they exhibit before solidification causes the material of the cement to be drawn strongly by capillary action into the clearance space surrounding the shading coil loops 48 and into the minute spaces between laminations. When the cure is completed, the yoke assembly 43 becomes bound together to act as a single, unitary piece. It is desirable, therefore, to precisely position the laminations before heating to cure the cement 49, thus producing a dimensionally precise yoke.

Among the advantages of rendering the yoke 43 rigid, in the manner'mentioned, is the excellent insulation which is provided between laminations. Because of the rigidity of the structure thus formed, excessive riveting pressures with attendant rupture of the oxide film can be avoided and varnishing of the individual laminations need not be resorted to. At the same time, the shading coils 48 are retained more securely than is the case where upsetting or staking is resorted to and, on this account, the insulation of the shading coil is improved so that its effect will be more precisely predictable. Avoidance of heavy cold working of the metal of the shading coils is an advantage from the standpoint of strength. As a result, the likelihood of loosened or broken shading coils is greatly diminished when the same are cemented in place, asindicated in Fig. 9.

It is desirable to employ a cementing composition which is semi-rigid rather than hard and brittle and certain compositions employing epoxy resins, formulated to exhibit semi-rigid properties, are preferred, although any suitable cementing substances may be employed.

The armature 44 is made up of side plates 51 and laminations held between by means of, rivets, as shown in Fig. 4, and has upwardly facing pole ends slotted at 50 opposite the shading coils 48 to avoid formation, on repeated closing, of ridges which, under slight misalignment, might interfere with sealing. The side plates 51 of the armature 44 are supplemented by lower shoulder plates 52 on opposite sides of the armature 44 which extend beneath inward projections 53 formed in the side arms 29 of carrier 26. Projections 53 are shaped, as shown in Fig. 11, to provide a flat central bearing area to ensure centralized application of force to the carrier and maintain the armature parallel to the yoke 43.

The entire armature is urged upwardly and strongly held by a leaf spring 54 positioned above the horizontal connecting member 55 of the carrier 26. A shim 56, inserted to tension spring 54, is formed of insulating material so as to also electrically separate the bottom of armature 44 from the spring 54, thus avoiding electrical short circuiting of the laminations by the spring 54. The shim 56 may serve the additional purpose of retaining the armature 44 against inward and outward displacement. This is accomplished through the engagement of of a small upward projection 57 on spring 54 with a corresponding downwardly opening hole 58 in the shim 56 and through engagement of the upper face of the shim 56, with a shallow recess between downward projections 6969 on the lower face of armature 44.

When the switch is in open position, as shown in Fig. 4, the armature 44 rests upon a shoulder 59 on the cap piece 38 and upon a ledge piece 60 staked into the frame 1, as shown in Fig. 4. The ledge piece 60 and the shoulder 59 of the cap piece 38 may be composed of or faced with a yieldable material capable of absorbing shock and thus minimizing peak stresses due to impact at the end of the opening stroke.

The coil The magnet is excited by a double winding coil composed of a winding 61 surrounding the pole end 62 of yoke 43 and a winding 63 surrounding the pole end 64 of yoke 43. The windings 61 and 63 are relatively short, extend to and overlap the working air gaps between the poles 62 and 64 and the corresponding parts of the armature 44 and completely surround the working gaps just prior to arrival of the armature in sealed position. In terms of the pole face area, the turns of the windings 61 and 63 are all relatively close to the working gaps thus minimizing leakage flux, reducing fringing and increasing efiective pull force, particularly in the latter part of the closing motion.

The windings 61 and 63 are formed into an integral coil assembly by encasing the same in insulating material forming a shell 65 of approximately rectangular form having two vertical rectangular openings occupied by the iron parts of the magnet. Beneath the windings 61 and 63, and if desired, embedded within the shell 65, as shown, is a magnetic shunt plate 66 of roughly figure 8 configuration but with an interruption 67 in the central portion thereof. The shunt plate 66 acts to diminish the effective pull force in open position without materially detracting from attractive force at and in the vicinity of scaled position, thus (in switches with heavy springs 22) insuring closure of the switch with full compression of the springs 22 under any condition of exciting voltage high enough to cause pick-up to occur.

The coil 45 is preferably formed and encased by a so-called potting or molding technique which produces intimate physical contact between the shell 65 and the windings 61 and 63, thus improving transport of heat from the windings proper to the surface of the coil. The coil 45 being in rectangular form closely fits the pocket 6 in the frame 1 and thus rapidly and readily delivers heat from the windings 61 and 63 to the frame 1. The upper surface of the coil 45, furthermore, is closely adjacent the projections 2 and 3 of the frame 1 and these act further to extract heat originating in the windings 61 and 63. The coil 45, consequently, operates at a lower tern perature than would be the case if it did not deliver heat readily to the frame 1.

Operation In operation, the armature 44 is strongly urged by the spring 54 into a position accurately related to the carrier 26. The carrier 26, in turn, is held against rocking movements by the widely spaced slide surfaces previously described. As a result, during movement of the armature 44 towards closed position, it is strongly restrained against tilting sutficiently to materially unbalance the attractive forces in the two working gaps. At the moment of engagement of the armature 44 with the yoke 43, however, the spring 54 may slightly compress thus relieving contact between the shoulder plates 52 and the projections 53 permitting a momentary adjustment of the armature 44 which greatly aids sealing. At the same time, the yoke 43, on account of the yielding character of projections 46 and 47, may move slightly to further accommodate any minute misalignment at the moment of sealing. The magnet thus comes to a completely sealed state.

Both the yoke 43 and the armature 44 are provided with pole ends which are chamfered slightly, as indicated at 68 in Fig. 4. Such chamfering augments the flux density in the sealed pole surfaces and in this way increases the attractive force without substantially increasing the total reluctance of the magnetic circuit.

There is. thus provided a switch in which the actuating magnet will develop adequate actuating force but which will occupy reduced space, thus producing a more compact structure without sacrifice of performance. It has been further found that switches, so constructed, exhibit exceptionally long service life.

I claim:

1. In an electromagnetically actuated switch the combination comprising a mounting frame formed of a metal having a high coefficient of thermal conductivity and including a pair of spaced outstanding posts of substantial cross-section; stationary contact mounting insulating means secured to said frame; a movable contact carrier slidably mounted on said frame; movable contacts secured to said carrier engageable with said stationary contacts; and a magnet oper-ati-vely connected to said movable contact carrier for moving the same, including a magnetic yoke mounted on said frame with a substantial portion thereof fitting snugly between said posts to be. in heat transmitting relationship thereto, a coil havingwindings mounted in exciting relation to said yoke and with a side thereof adjacent said posts thereby being in. heat transmitting relationship to said frame, and a mag netic armature movable to and from a sealed position with respect to said yoke connected to said carrier to actuate the same and to move said movable contacts; into engagement with said stationary contacts.

2. In an electromagnetically actuated switch in ac-- cordance with claim 1 wherein the movable contact carrier is non-metallic for dry bearing engagement with said metallic frame, and includes a pair of arms that slide against said posts for guided movement which arms extend downwardly from the posts for engagement with the armature.

3,. In an electromagnetically actuated switch in accord-- ance with claim 1 wherein the mounting frame is formed of an aluminum composition having a high coeflicient: of thermal conductivity.

4. In an electromagnetically actuated switch in accordance with claim 1 wherein the movable contact carrier, slidably mounted on said frame, is composed of a fibre reinforced thermosetting resin.

5. In an electromagnetically actuated switch having stationary contacts and movable contacts; a frame; a magnet for moving said movable contacts comprising a yoke, a coil and a movable armature; a slidable carrier including armature guiding means secured to said arma;

ture and movable contact supports connected to said movable contacts for moving the latter; and mounting means on said frame furnishing support for said coil and said yoke, said mounting means including adjacent opposing grooves in the yoke and the frame together with a tongue inserted in and extending between the grooves thereby yieldably supporting said yoke, said armature, said movable contacts and said carrier when said magnet is in closed position.

6. in an electromagnetically actuated switch in accordance with claim wherein the frame is composed of metal of high thermal conductivity with a pair of protruding posts horizontally spaced from one another which extend closely alongside opposite sides of said yoke and 'are each closely adjacent said coil to be in heat transmitting relationship thereto.

7. In an electromagnetically actuated switch in accordance with claim 5 wherein the tongue between the grooves is a yieldable impact absorbing part opposing displace ment of the yoke by the forces resulting when the armature strikes the yoke on closing.

8. In an electromagnetically actuated switch in accordance with claim 5 wherein a leaf spring is inserted between the armature and the slidable carrier to urge the armature upwardly with respect to the carrier.

9. in an electromagnetically actuated switch the com bination comprising a frame; longitudinal metallic guide ways on said frame; movable contact carrier means having slide surfaces engaging the guide ways on said frame, said slide surfaces being composed of non-metallic material for dry bearing engagement with said longitudinal guide ways; an eiectromagnet having a yoke and windings mounted on said frame and an armature movable toward and away from said yoke; a connection between said armature and said movable contact carrier for transmitting movement of said armature to said carrier, which connection comprises a leaf spring between the armature and the contact carrier which is engageable with the carrier to prohibit lateral displacement of the spring with respect to the carrier upon assembly, an insert between said armature and spring that loads the spring upon assembly to enhance the spring force upon the armature, and an engagement between the armature and the carrier to the side of the armature opposite said spring to thereby restrain lateral motion of the armature with respect to the carrier; and movable contacts mounted on said carrier to be moved thereby into and out of engagement with said stationary contacts.

10. In an electromagnetically actuated switch the combination comprising a frame, a pair of horizontally spaced inner guide ways standing out from said frame and a pair of horizontally spaced outer guide ways on said frame which are beneath said inner guide ways, movable contact carrier means having upper and lower slide surfaces engaging the guide ways on said frame with the upper slide surfaces passing to the outside of said inner guide ways and the lower slide surfaces passing to the inside of said outer guide ways, an electromagnet having a yoke with downwardly facing pole surfaces mounted on said rame with a portion thereof between the inner guide ways and an armature with upwardly facing pole surfaces movable toward and away from said yoke, windings supported by said frame, means forming a connection between said armature and said movable contact carrier locating the pole surfaces of the armature between said upper and lower slide surfaces for transmitting movement of said armature to said carrier, and movable contacts mounted on said carrier to be moved thereby into and out of engagement with said stationary contacts.

11. In an electromagnetically actuated switch the combination comprising a heat conductive metallic frame, longitudinal guide ways on said metallic frame, movable contact carrier means having slide surfaces engaging the guide ways on said frame having a recessed'inner face and oppositely disposed side faces, a coil socket in said frame, an electromagnet having a yoke and a coil seated in the coil socket in said frame with each being snugly receivedbetween said side faces and an armature movable'toward and away from said yoke, means forming a connection between said armature and said movable contact carrier for transmitting movement of said armature to said carrier, a pair of heat absorbing metallic projections on said frame flanking said yoke-and movable contactsmounted on said carrier to be moved thereby into andout of engagement with said stationary contacts.

12. in an electromagnet combination comprising a U shaped yoke having a pair of legs with pole faces at the ends of the legs, a pair of windings one surrounding each leg and extending beyond the same, an-armature having a pair of pole ends facing the pole faces of said yoke movable toward and away from sealed relation with said yoke, and a magnetic shunt member bridging the outer ends of said windings furnishing a path for flux supplementing the path provided by the armature in open position to'increase the pick-up voltage without material reduction ofat-tractive force at and near sealed position.

13. In a magnetic member having embedded electrical conductor means, a compacted, laminated, magnetic'body made up of a plurality of adjacent magnetic laminations spaced over parts'of the areas thereof by thin interlamellar voids resulting fromnon-planar surfaces remaining on the laminations after compaction, said body having a slot 'ex tending transversely of said laminations and communicati'ng with'said interlamellar voids; electrical conductor 'means disposed in said slot and filling the same saving a clearance sp'ace'therebetween; and means uniting thelaminations of the member and uniting the member and conductor means comprising cured, eleetrieally-insulating, thermosetting epoxy resin disposed in and cured in 'situ in the interlamellar spaces of the laminations and in the conductor slot clearance spaces.

14. In an electromagnetic actuator having a magnetic yoke, a coil associated with said yoke, an actuator, and an armature joined with the actuator the improvement of a connection between the armature and the actuator comprising an abutment on the actuator disposed to one side of the armature to limit armature movement, a spring inserted between the armature and the actuator at the opposite side of the armature to urge the armature toward said abutment, and an insert disposed between the armature and spring that aids loading of the spring.

15. In an electromagnetic actuator the combination comprising a mounting frame of a metal of high thermal conductivity having a pair of heat conductive pillars standing outwardly therefrom, a magnet having a U- shaped yoke and a U-shaped armature to present a pair of working gaps with a portion thereof inserted snugly between the conductive pillars to pass heat thereto, and a composite coil comprising a pair of windings each surrounding a working gap and being closely adjacent said pillars in heat transferring relation thereto.

References Cited in the file of this patent UNITED STATES PATENTS 1,488,134 Richmond Mar. 25, 1924 1,691,203 Lee et al Nov. 13, 1928 2,378,162 Stapleton June 12, 1945 2,411,374 Horstrnan Nov. 19, 1946 2,434,096 Ayers et al. Jan. 6, 1948 2,481,431 Logan Sept. 6, 1949 2,528,932 Wiles et al. Nov. 7, ,1950 2,579,560 Ford Dec. 29, 1951 2,623,920 Ford Dec. 30, 1952 2,687,502 Furnas et al. Aug. 24, 1954 2,692,314 Lawrence Oct. 19, 1954 FOREIGN PATENTS 640,233 France Mar. 24, 1928 UNITED STATES PATENT OFFICE CERTIFICATE @F CURRECTION Patent No. 2,903,537 September 8, 1959 Leland l3a Lawrence of the above numbered patent re Patent should read as corrected below.

Column '7, lines '74 and '75, strike out "having a recessed inner face and oppositely disposed side faces" and insert the same after "frame" and before the coma in column 8, line 1 Signed and sealed ohis 12th day of April 1960.,

SEAL) ttest:

KARL H. vAXLINE ROBERT C. WATSON Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No'. 2,903,537 September 8, 1959 Leland E, Lawrence ppears in the -printed specification that the said Letters It is hereby certified that error a of the above numbered patent requiring correction and Patent should read as corrected below.

Column '7, lines 74 and '75, strike out "having a recessed inner face and oppositely disposed side faces" and insert the same after "frame" and before the coma in column 8, line 1.

Signed and sealed this 12th day of April 1960.

SEAL) ttest:

KARL H, .AXLINE Attesting Officer ROBERT C. WATSON Commissioner of Patents 

